def IsPrime(a):
    if a==1: return False
    test= True
    k=a-1
    while k>1:
        if not a%k:
            test=False
            break
        k-=1
    return test

n=input()
r=[]
for i in range(len(n)):
    for k in range(i + 1, len(n) + 1):
        a = int(n[i:k])
        if IsPrime(a):
           r+=[a]
print(max(r) if r else 0)
